This invention relates generally to fiber-resin composite pultrusion methods and products and, more particularly, to rods having one or more longitudinally extending reinforcing beads, and to a method for pultruding such rods.
In manufacturing a handle for a hand tool such as a shovel, competing design considerations are at play. On the one hand, it is desirable to have a handle that is as light as possible, to provide for easy use by consumers. On the other hand, the handle must have the structural integrity to withstand the variety of stresses that will be placed on it. Wooden handles have been used, but these provide an unacceptable compromise of weight for structural integrity or vice versa.
An alternative to wooden handles is the use of rods formed from resin coated fibers. The basic technique for running filaments through a resin bath and then into a long heated die tube to produce a cured composite of the same shape as the die tube has been known for some time. See, for example, U.S. Pat. Nos. 2,948,649 and 3,556,888. This method, however, produces a solid extruded product which is unacceptably heavy and/or too rigid.
The weight problem can be alleviated by means of an existing process to extrude hollow tubes utilizing a die tube with the center filled, leaving an annular cross-section through which the fibers are pulled. However, this weight reduction is achieved at the cost of significantly lower tensile and compression (bending) strength than that of a solid rod, and therefore would not be suitable for use in certain high-stress applications, such as general purpose shovel handles. Further, to increase interlaminar strength, a substantial percentage of fibers running other than in a longitudinal direction are usually required. Moreover, commercial machines for producing continuous hollow tubing are extremely expensive.
As illustrated and described in my co-pending U.S. patent application, Ser. No. 407,818, the bending strength of tool handles can be improved by producing rods which are substantially hollow or lightweight throughout most of their length, but reinforced at areas of expected increased stresses. However, even such improved tool handle rods have proven to be less than ideal in some situations due to the inherent strength characteristics of the fiber-resin composite material forming the rod jackets. For instance, the edge of a hole often acts as fulcrum upon the bottom side of the shovel handle when the shovel head is firmly lodged in the hole and downward pressure is exerted on the handle. In such situations, extreme bending stresses are created within the handle near the fulcrum point, tending to compress the bottom side of the handle and pull-apart the top side.
When a tool handle constructed of a fiber-resin composite fails under such loading, the failure usually occurs along the bottom side because fiber-resin composite materials are typically much weaker in compression than in tension. Moreover, depending on the depth of the hole and the manner in which the downward pressure is applied, the fulcrum point against the bottom side of the handle will vary widely in different situations. It is therefore difficult to adequately strengthen a shovel handle with reinforcing cores alone, since the maximum stresses can be applied anywhere along the entire length of the handle.
Accordingly, there has existed a need for a lightweight tool handle rod having superior strength characteristics than prior handles, and a convenient method for producing such a rod in a quick, reliable and efficient manner. A reinforced rod having increased resistance to failure in compression, while minimizing any weight increase, is preferred. Further, it is desirable that such tool handle rods have improved bending resistance along their entire lengths, since the points of greatest stress can vary greatly. The present invention satisfies these needs and provides other related advantages.